1. Technical Field
The present disclosure relates to a fuel cell system.
2. Description of the Related Art
A fuel cell system is provided with: a reformer that subjects a raw material such as natural gas or LPG to a reform reaction together with water vapor to form a hydrogen-containing fuel gas; and a fuel cell that uses the fuel gas produced by the reformer and oxygen in an oxidant gas to generate power.
The fuel gas and oxidant gas that is not used to generate power by the fuel cell is combusted and becomes exhaust gas, and the heat of the exhaust gas is used for heating the reformer and the fuel cell. In addition, the exhaust gas is cooled by a heat exchanger or the like and the water vapor in the exhaust gas is recovered as condensed water.
The condensed water recovered is subjected to purification treatment with an ion exchange resin or the like and is once again used as reforming water for a reform reaction.
In this way, a fuel cell system is known in which a water self-sustaining operation is able to be carried out by recovering the water vapor in exhaust gas as condensed water without reforming water being supplied from outside.
A method in which water is used for the heat-receiving fluid of a heat exchanger is common as a method for cooling exhaust gas with a heat exchanger. Thus, a fuel cell system is known which is provided with a water storage tank that stores hot water that is output from a heat exchanger, and a heat recovery path by which water circulates through the water storage tank and the heat exchanger. However, in this fuel cell system, when the water storage tank is filled with hot water of a high temperature, the hot water flows to the heat exchanger as it is and the exhaust gas is not able to be cooled. Therefore, in this situation, there is a risk of it becoming difficult to recover condensed water.
Thus, a method has been proposed (Japanese Unexamined Patent Application Publication No. 2010-277973, for example) in which a water level detector that detects the water level of a condensed water tank is provided as a way of determining the success or failure of a water self-sustaining operation, and in the case where it is detected that the level of the condensed water is low, the output of the fuel cell system is decreased regardless of the power demand to reduce the amount of heat of the exhaust gas and increase the amount of circulation water. Thus, Japanese Unexamined Patent Application Publication No. 2010-277973 discloses that it is possible to ensure the amount of condensed water that is recovered.
Furthermore, a method has also been proposed in which, when a water self-sustaining operation fails, the output of a fuel cell system is increased in addition to the power demand by activating an auxiliary machine in the fuel cell system (Japanese Unexamined Patent Application Publication No. 2013-222577, for example). Thus, Japanese Unexamined Patent Application Publication No. 2013-222577 discloses that the amount of condensed water recovered is able to be increased.